COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      Number 902

                                     Konkoly Observatory
                                     Budapest
                                     1974 June 28



              NEW PERIODS FOR ST Pic AND XZ Cet

    In many variable stars the period may be incorrectly determined 
if observations are only obtained once a night. This occurs since
several integral periods may elapse during a 24 hour period. The true
period P_n will be one of the family 1/Pn = 1/P_{obs}-n, where P_{obs} is
the apparent period in an integer. ST Pic, one of about twenty
cepheids observed at SAAO in 1973 is described in the General Catalogue 
of Variable Stars, GCVS, as a 9th Magnitude C_{delta} of 18.75 day
period. One spectrum was taken each night during several observing
sessions using a grating spectrograph on the 40-inch reflector, and
simultaneous V, B and I photometry was obtained on the 20-inch.

    Since the spectra appeared rather too early in type, and the colour 
indices appeared too blue for a classical cepheid, it was decided
to observe the star several times during one night. A change of 0.25
magnitudes was obtained in V over an interval of 3 hours, indicating
that ST Pic is a short period variable. The available photometry suggests 
0.486 days as the most likely period. We hope to complete the
visual light curve, and confirm the period next summer.

    XZ Cet, described in GCVS as a 9th magnitude RR Lyrae variable
of 0.451 day period was observed spectroscopically and also in V,B,
I on 14 nights between August and December 1973. On plotting the light
curve it was found that the portions of curve derived from runs taken
on certain nights disagreed in phase with the light curve derived on
other nights. On investigations, one of the "reciprocal" periods of
0.45 days, 0.823 days, was found to give a smooth light curve with no
discrepancies. The radial velocity curve appears to fit both periods
equally well. It is possible that the discrepancies in the 0.451 day
light curve are due to arbitrary phase shifts rather than an incorrect
period. To check this photometric observations will be taken 12 hours
apart when XZ Ceti is suitably placed.

    I would like to thank L. A. Balona, B. D. Kelly and the photometric
department for help with the observations.


South African Astronomical                           J. F. DEAN
Observatory P.O. Box 9.
Observatory, C.P.
7935
South Africa